Streptococcal infections in horses are mainly caused by the species Streptococcus equi, which comprises three subspecies designated equi, zooepidemicus, and ruminatorium respectively hereafter simply called S. equi, S. zooepidemicus and S. ruminatorium (Refs. 15, 24, 40).
S. equi which is virtually confined to horses is the causative agent of strangles, a world-wide distributed and highly contagious serious disease of the upper respiratory tract of the Equidae. Strangles is one of the most frequently reported equine diseases world-wide and is characterized by fever, nasal discharge, and abscess formation in the retropharyngeal and mandibular lymph nodes. In some cases the disease shows a metastatic course in the body, so called “bastard strangles”. The disease has a world-wide distribution and causes great economic losses (Ref. 39).
S. zooepidemicus is considered as an opportunistic commensal often occurring in the upper respiratory tract of healthy horses. However, after stress or virus infection, it can cause a secondary infection, which results in strangles-like symptoms. Moreover, S. zooepidenzicus infects not only horses but also a wide range of other animals, like pigs, goats, dogs, cats, and cows. Even human cases of infection due to subsp. zooepidemicus have been reported (Ref. 5). This subspecies has been implicated as the primary pathogen in conditions such as endometritis, cervicitis, abortion, mastitis, pneumonia, abscesses and joint infections.
S. ruminatorium has been isolated from milk of sheep and goats with mastitis (Ref. 10).
Streptococcus pyogenes is an important human pathogen which causes a variety of diseases e.g. impetigo, pharyngitis, necrotizing fasciitis and toxic shock syndrome.
Although it is possible to treat and cure these streptococcal infections with antibiotics, such as penicillin, tetracycline or gentamicin, an effective prophylactic agent that could prevent outbursts of such infections and obviate or reduce the risk for development of resistant strains associated with antibiotic treatment, would be appreciated.